Fig. 4.

Model. The spliceosome’s catalytic core can be found in both open and closed conformations. Prp8 is shown (tan blob) with the 740 loop (green), U6 snRNA (red), U5 snRNA (blue), pre-mRNA (black), and Mg²⁺ (magenta). (A) When the catalytic core is open, there is no pre-mRNA docked to U5 snRNA, catalytic Mg²⁺ are absent, and the 740 loop is unresolved. (B) Upon closure, the large domain of Prp8 rotates with respect to the Prp8 NTD (top of Prp8 moves toward bottom). The Prp8 740 loop forms and is resolved by cryoEM (SI Appendix, Fig. S2). The 740 loop supports the U6 internal stem loop, which when in a closed conformation, coordinates Mg²⁺. Pre-mRNA base pairs with the U5 snRNA. (C) We hypothesize prp-8 alleles negatively affect closure, disrupting pre-mRNA positioning and the 740 loop. prp8 spliceosomes would allow movement of pre-mRNA within their catalytic core, which could result in cryptic splicing.